4.2 Indicators and pH
Summary: A student has two beakers. One contains an acid and the other contains an alkali. How can they find out which is which? Tags: science oxford-science student-book-7 chemistry Created: 2026-05-19 Last Updated: 2026-05-19
After this topic, you will be able to:
Think back
- Give one property of an acid.
- Which has the greater number of alkali particles per litre – a concentrated or dilute solution?
- How can you control the risks of using a corrosive solution?
Key idea Indicators tell us if a solution is acidic or alkaline. Universal indicator uses the pH scale to measure how acidic or alkaline a solution is.
Key words indicator, litmus, universal indicator, pH scale, neutral
A student has two beakers. One contains an acid and the other contains an alkali. How can they find out which is which?
You can use an indicator to find out whether a solution is acidic or alkaline. An indicator contains a dye (colour). The dye turns a different colour in acidic and alkaline solutions.
Which plants make good indicators?
You can make indicators from plants. Table 1 gives the colours of juices from three plants in acidic and alkaline solutions. Table 1 The colours of some plant indicators in acidic and alkaline solutions. Juice from …
Colour in dilute hydrochloric acid
Colour in dilute sodium hydroxide (an alkali)
red cabbage
red
yellow/green
hibiscus flower
dark pink/red
dark green
beetroot
red/purple
yellow
A A student adds beetroot juice to an alkaline solution. Predict the colour of the mixture.
At school, you might use litmus indicator. Litmus is a solution of dyes from lichens. Paper can be soaked in litmus solution to make litmus paper.
- Red litmus turns blue in alkaline solutions.
- Blue litmus turns red in acidic solutions (see Figure 1).
Figure 1Using litmus paper.
B A student adds a solution to litmus paper. There is a colour change from red to blue. State whether the solution is acidic or alkaline.
How acidic? How alkaline?
Which is more acidic: vinegar or stomach acid? How can you find out? You cannot use blue litmus paper. Both acids would make it red.
Instead, you need universal indicator. Universal indicator is a mixture of dyes. It changes colour to show how acidic or alkaline a solution is. Figure 2 shows these colours.
Figure 2Universal indicator changes colour depending on the pH. Full text alternative to the content
What is the pH scale?
The pH scale is a measure of how acidic or alkaline a solution is. On the pH scale:
- an acid has a pH of less than 7 – the lower the pH, the more acidic the solution
- an alkaline solution has a pH of more than 7 – the higher the pH, the more alkaline the solution.
Some solutions are neutral. This means they are neither acidic nor alkaline. The pH of a neutral solution is exactly 7.
Universal indicator is a different colour at each pH. The scale in Figure 2 shows the colours of universal indicator in solutions of different pH.
C Look at Figure 2. Name two useful things that are acidic. Name two useful things that are alkaline.
Table 2 Amie collected the data in Table 2. Use the data to list the names of the solutions in order of increasing acidity, starting with the least acidic. Table 2 The pH values of five solutions. Solution pH milk 6.6 orange juice 3.2 black coffee 5.5 lemon juice 2.3 vinegar 2.8
Stretch zone
- Copy and complete the sentences. Adding an acid to red litmus paper makes the litmus paper go red/blue. On the pH scale, acids have a pH of less/more than 7. The higher the pH, the more/less acidic the solution. A solution is alkaline if its pH is more/less than 7. A neutral solution is of pH 7/0.
- Universal indicator gives the pH of a solution. A student adds universal indicator to solution X. The resulting mixture is purple. State whether solution X is acidic, alkaline, or neutral.A solution has a pH of 4. Is the solution acidic, alkaline, or neutral?
Stretch zone
- John has a solution. It turns yellow when he adds juice. Predict the colour of the solution if he added hibiscus flower juice. Explain your answer.